Honeycomb heater having integrally formed electrodes and/or integrally sintered electrodes and method of manufacturing such honeycomb heater

ABSTRACT

A honeycomb heater having integrally formed and/or integrally sintered electrodes constructed of a metallic honeycomb structure having a desired honeycomb configuration, and metallic electrodes which are integrally sintered with the honeycomb structure at predetermined positions thereon. The honeycomb heater having integrally sintered electrodes is manufactured by joining or contacting the electrode-like formed bodies to the formed honeycomb body, and then by sintering the formed honeycomb body with the electrode-like formed bodies attached or contacted thereto. The electrodes are made of the same material as that of the honeycomb body, or of a material having an electric resistance lower than that of the material comprising the honeycomb body. Alternatively, the honeycomb heater having integrally formed and integrally sintered electrodes is manufactured by preparing an extrusion of a honeycomb body as one body which has a honeycomb configuration and an electrode-like shape at predetermined positions thereon, and then sintering that formed honeycomb body.

This is a continuation of application Ser. No. 07/581,985 filed Sep. 14,1990 now U.S. Pat. No. 5,200,154.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb heater having integrallyformed and/or sintered electrodes and employing a honeycomb structure,and a method of manufacturing such a honeycomb heater.

2. Description of the Related Art

Conventionally, porous ceramic honeycomb structures have been employedas catalysts or substrates for catalysts which remove, for example,nitrogen oxides, in the exhaust gas of internal combustion engines, suchas automobiles.

In recent years, there has been a desire to develop materials exhibitinggreater mechanical strength and thermal resistance in hostileenvironments. To these ends, honeycomb structures manufactured byforming metal powders and then sintering the formed body have been putinto practical use.

The present inventors have already proposed in U.S. patent applicationSer. Nos. 07/545,509, now U.S. Pat. No. 5,63,029, and 07/545,697, nowabandoned, a honeycomb heater which is comprised of a honeycombstructure with electrodes formed thereon to supply an electric currentto the honeycomb structure. This honeycomb structure is produced byforming ceramic or metal powders into a desired honeycomb configurationand then sintering the formed honeycomb body.

In the above-described honeycomb heater, the electrodes are mounted onthe outer peripheral wall or inside of the honeycomb structure by theconventional welding or brazing.

However, in the case where the electrodes are mounted on the outerperipheral wall of the honeycomb structure by welding, the portion ofthe thin outer peripheral wall of the honeycomb structure on which theelectrodes are mounted is subjected to heat generated by the welding andmay thus be damaged. Brazing requires a large number of manhours and isexpensive. Furthermore, in the above-described honeycomb heater, sincethe electrodes are joined to the sintered honeycomb structure, thejoined portions are exposed to heat in the exhaust gas during the use,deteriorating the durability of the honeycomb heater.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a honeycomb heaterhaving integrally formed and/or integrally sintered electrodes whicheliminates the aforementioned problems associated with theabove-mentioned techniques, and to provide a method of manufacturingsuch a honeycomb heater.

To this end, the present invention provides a honeycomb heater havingintegrally formed and sintered electrodes which comprises a honeycombstructure having a desired honeycomb configuration, and electrodes whichare integrally formed with the honeycomb structure at predeterminedpositions thereon. The electrodes are made of the same material as thatof the honeycomb structure or of a material having an electricresistance lower than that of the material comprising the honeycombstructure.

The present invention also provides a method of manufacturing ahoneycomb heater having integrally sintered electrodes by powdermetallurgy. This method of manufacturing comprises the steps ofpreparing a formed honeycomb body by forming raw material powders into adesired honeycomb configuration and preparing electrode-like formedbodies made of the same material as that of the formed honeycomb body orof a material having an electric resistance lower than that of thematerial of the formed honeycomb body. The electrode-like formed bodiesare then joined to the formed honeycomb body at predetermined positionsand the resulting formed honeycomb body with the electrode-like formedbodies joined thereto is sintered together.

The present invention further provides a method of manufacturing ahoneycomb heater having integrally formed and sintered electrodes bypowder metallurgy. This manufacturing method comprises the steps ofpreparing a formed honeycomb body as one body which has anelectrode-like shape at predetermined positions by forming raw materialpowders into a honeycomb configuration having a large number ofpassages, and then sintering the resultant formed body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an example of a honeycomb structuremanufactured in the present invention; and

FIG. 2 is a plan view of another example of the honeycomb structuremanufactured in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the first aspect of the present invention, the honeycomb heater ismanufactured by joining or contacting the electrode-like formed bodiesto the formed honeycomb body, and then by sintering the formed honeycombbody with the electrode-like formed bodies attached or contactedthereto. The electrodes are made of the same material as that of thehoneycomb body, or of a material having an electric resistance lowerthan that of the material comprising the honeycomb body.

In the second aspect of the present invention, the honeycomb heater ismanufactured by preparing a honeycomb body as one body which has ahoneycomb configuration and which has an electrode-like shape atpredetermined positions thereon by extrusion or the like, and thensintering that formed honeycomb body.

Thus, honeycomb heaters having integrally formed electrodes atpredetermined positions on the honeycomb structure can be effectivelymanufactured without damaging the honeycomb structure. In this case, theelectrodes are made of the same material as that of the honeycombstructure, or a material having an electric resistance lower than thatcomprising the honeycomb structure. Furthermore, since the portion ofthe honeycomb structure joined to the electrodes is continuous in termsof material, the resultant honeycomb heater exhibits excellentdurability in high-temperature environments.

Whereas any material, ceramic or metal, capable of generating heat whenenergized, can be used as the material of the honeycomb heater, the useof metal enhances the mechanical strength and is thus preferred.Examples of such metals include stainless steel and materials havingcompositions of Fe-Cr-Al, Fe-Cr, Fe-Al, Fe-Ni, W-Co, and Ni-Cr. Amongthe above materials, Fe-Cr-Al, Fe-Cr and Fe-Al are preferred because oflow cost and high resistance to heat, oxidation and corrosion.

The honeycomb structure employed in the present invention may be porousor non-porous. In the case where a catalyst is carried on the honeycombstructure, however, a porous honeycomb structure is preferred because acatalyst layer can closely adhere to such a honeycomb structure, anddoes not peel from the honeycomb structure even when a difference in thethermal expansion between the honeycomb structure and the catalystexists.

The metal honeycomb body will be prepared in the manner described below.First, Fe powder, Al powder and Cr powder, or alternatively powders ofalloys of these metals, are mixed to prepare a metal powder mixturehaving a desired composition. Subsequently, the metal powder mixture isblended into an organic binder, such as methyl cellulose orpolyvinylalcohol, and water to produce a readily formable mixture. Thatmixture is then formed into a shape of a desired honeycomb configurationby extrusion, pressing or casting.

The thus-obtained formed honeycomb body may be of the form of ahoneycomb body which is formed into a honeycomb configuration having alarge number of passages and which has an electrode-like shape atpredetermined positions thereon, or of a honeycomb body withelectrode-like formed bodies joined to predetermined positions thereon.

In the latter case, the electrode-like formed bodies are made of thesame material as that of the formed honeycomb body, or of a materialhaving an electric resistance lower than that of the material of theformed honeycomb body. Such electrode-like formed bodies are prepared inthe similar manner to that in which the honeycomb body is prepared.

Next, the formed honeycomb body which has an electrode-like shape at thepredetermined positions thereon, or the formed honeycomb body with theelectrode-like formed bodies joined to the predetermined positionsthereon, is fired in a non-oxidizing atmosphere at a temperature rangingbetween 1000° and 1450° C. During the sintering in the non-oxidizingatmosphere containing hydrogen, the organic binder is decomposed andthereby removed with the aid of Fe or the like, which acts as acatalyst. A good sintered body (a honeycomb structure) can therefore beobtained.

Sintering is not adequate at a temperature lower than 1000° C. Sinteringconducted at a temperature higher than 1450° C. causes deformation ofthe resulting sintered body and is thus undesirable.

Preferably, a heat-resistant metal oxide layer is then formed on thesurface of the cell walls and the surface of the pores of the obtainedhoneycomb structure.

Next, a resistance adjusting mechanism of a desired form is provided onthe obtained honeycomb structure between the electrodes thereof.

The resistance adjusting mechanism provided on the honeycomb structuremay take on any of the following forms:

(1) a slit or slits of any length, formed in any direction at anyposition

(2) variations in the length of the cell walls in the axial direction ofthe passages

(3) variations in the thickness (wall thickness) of the cell walls ofthe honeycomb structure or variations in the cell density of thehoneycomb structure, or

(4) a slit or slits formed in the cell wall (rib) of the honeycombstructure.

In the honeycomb heater having the integrally formed electrodesaccording to the present invention, the resistance thereof will bepreferably held between 0.001 Ω and 0.5 Ω.

The term, "a formed honeycomb body" is employed in this application torefer to an integral body having a large number of passages partitionedby the walls. The passages may have any cross-sectional form (cellshape), e.g., a circular, polygonal or corrugated form.

The present invention will further be illustrated in the followingexamples which are intended to be illustrative, but not limiting, ofthis invention.

EXAMPLE 1

Fe powder, Fe-Cr powder, and Fe-Al powder were mixed to prepare amixture having a composition of Fe-20Cr-5Al (% by weight), and theobtained mixture was then formed by the extrusion into a honeycomb bodyhaving an outer diameter of 105 mmφ, a length of 20 mm, a rib thicknessof 8 mil and square passages 12 at a passage density of 300 cells/in².The extruded honeycomb body was dried to obtain a honeycomb dried body10 shown in FIG. 1. Next, four plates 13, each having a thickness 2 mm,a width of 20 mm and a length of 60 mm, were prepared by the extrusionusing the same mixture. The formed plates 13 were attached to thehoneycomb dried body 10 with two of them making one pair, as shown inFIG. 1.

A slurry of the mixture having the same composition was used as theadhesive. After the adhesion, the adhesive was dried to obtain a driedhoneycomb heater having the integrally formed electrodes. Thereafter,the dried honeycomb heater having the integrally formed electrodes wasfired in H₂ atmosphere at 1300° C. Thereafter, slits 11 were formedadequately, and a honeycomb heater 14 with the electrodes formedintegrally thereon, having an outer diameter of 90 mmφ and a length of17 mm, was thereby obtained.

EXAMPLE 2

The powder mixture having the same compositions as that of the powdermixture employed in Example 1 was formed into a shape of an electrode 20shown in FIG. 2 by the powder pressing. The obtained electrodes 20 wereattached to the same honeycomb dried body as that employed in Example 1using as an adhesive a slurry obtained by mixing a binder and water to apowder mixture having the low electric resistance composition ofFe-20Cr-5Al-5Cu (% by weight). The adhesive was dried to obtain a driedhoneycomb heater having the integrally formed electrodes. Thereafter,the dried honeycomb heater having the integrally formed electrodes wasfired in H₂ atmosphere at 1300° C. Thereafter, slits 11 were formedappropriately, and a honeycomb heater 21 with electrodes formedintegrally thereon was thereby obtained.

EXAMPLE 3

The powder mixture having the low electric resistance composition ofFe-20Cr-5Al-5Cu was formed into a shape of an electrode 20 shown in FIG.2 by the powder pressing. The obtained electrodes 20 were attached tothe same honeycomb dried body as that employed in Example 1 using as anadhesive a slurry obtained by mixing a binder and water to the samepowder mixture as that used to form the electrodes 20. The adhesive wasdried to obtain a dried honeycomb heater having the integrally formedelectrodes. Thereafter, a honeycomb heater 21 having the integrallyformed electrodes was manufactured under the same conditions as those ofExample 2.

EXAMPLE 4

A honeycomb dried body was manufactured in the same manner as that ofExample 1. A formable mixture body for manufacturing honeycombconfiguration was cut into block-like shapes and these block-shapedbodies were dried. Thereafter, these dried bodies were each cut into ashape of the electrode shown in FIG. 2. Thereafter, a honeycomb heaterhaving the integrally formed electrodes was obtained under the sameconditions as those of Example 2.

EXAMPLE 5

The same powder mixture as that employed in Example 1 was formed into ahoneycomb body which had an electrode-like shape at predeterminedpositions thereon using an extruder die capable of forming the wholebody shown in FIG. 1. The obtained formed honeycomb body was dried andsintered. Thereafter the slits were formed adequately to obtain ahoneycomb heater having the integrally formed electrodes.

EXAMPLE 6

A honeycomb heater having the integrally formed electrodes and acatalyst was produced by coating γ-alumina in which CeCO₂ was present ina proportion of 8 wt % on the honeycomb structure of the honeycombheater having the integrally formed electrodes which was obtained inExample 1, by loading 20 g/ft³ of Pd and Pt on the coated layerrespectively, sintering the whole structure at 600° C. and thenconnecting the electrodes to leads which were in turn connected to apower source.

The obtained honeycomb heater having the integrally formed electrodesand catalyst was provided in advance of (upstream of) a three-waycatalyst available on the market which was a main monolithic catalysthaving an outer diameter of 90 mmφ and a length of 80 mm (a ribthickness of 6 mil and a passage density of 400 cells/in²).

The performance of this system at the beginning of the operation of anengine was tested by introducing into this system an exhaust gas whosetemperature was raised at a same rate from 100° C. to 420° C. for twominutes and was then maintained at 420° C. for one minute (warming-uptest) and by measuring the conversion for CO, HC and NOx. Table 1 showsthe results of the measurements. At the beginning of the test, thehoneycomb heater having the integrally formed electrodes and catalystwas energized for one minute by a battery of 12 V in order to heat theexhaust gas to 350° C.

Next, the durability test was conducted on the above system in themanner described below: the temperature of an exhaust gas was raisedfrom the room temperatures to 750° C., and then maintained at 750° C.for ten hours. While the temperature was maintained at 750° C., a cyclicoperation was repeated in which the engine was cruised for 60 secondsand then introduction of the fuel was cut for 5 seconds. At thebeginning of the durability test, the honeycomb heater having theintegrally formed electrodes and catalyst was energized for one minutein the same manner as that in which it was energized in the abovemeasurements in order to heat the exhaust gas to 350° C.

After the above-described durability test was performed ten times, thestate of the honeycomb heater having the integrally formed electrodesand catalyst was observed. No defect was found in the electrodes or inthe vicinity thereof.

                  TABLE 1                                                         ______________________________________                                        Average conversion (%) (warming-up test)                                      CO              HC     NOx                                                    ______________________________________                                        63              50     63                                                     ______________________________________                                    

As will be understood from the foregoing description, it is possibleaccording to the present invention to efficiently and economicallymanufacture honeycomb heaters having the integrally formed electrodeswhose honeycomb structure is not damaged and which exhibits excellentdurability in high-temperature environments.

What is claimed is:
 1. A honeycomb heater, comprising:a metallicsintered honeycomb body; and sintered metallic electrodes disposed atpredetermined positions on said metallic honeycomb body, said metallicelectrodes being made of the same material as that of said metallichoneycomb body or of a material having an electric resistance lower thanthat of the material comprising the metallic honeycomb body, whereinsaid metallic electrodes and said metallic honeycomb body have beensimultaneously sintered.
 2. The honeycomb heater of claim 1, whereinsaid metallic honeycomb body and said metallic electrodes are integrallyformed and comprise one-piece.
 3. A method of manufacturing a honeycombheater having simultaneously sintered electrodes comprising the stepsof:preparing a formed metallic honeycomb body by forming metal powdersinto said formed metallic honeycomb body; preparing metallic electrodesmade of the same material as that of said formed metallic honeycomb bodyor of a material having an electric resistance lower than that of thematerial of the formed metallic honeycomb body; joining said metallicelectrodes to predetermined positions on said formed metallic honeycombbody; and sintering simultaneously the formed metallic honeycomb bodyand the metallic electrodes joined thereto.